From Nookipedia, the Animal Crossing wiki
The Salmon (サケ, Sake) is the most prolific member of the salmon family. It is a fish that appears during the 30 days of September. It appears in the River Mouth in the first half of September, and the river itself in the second half. In New Leaf, Salmon are found directly adjacent to the waterfall. The Salmon sells for 700 Bells. According to the City Folk fish encyclopedia: "These creatures split their lives between freshwater and seawater." In the Wild World museum, the fish is in the middle tank with its big brother, the King Salmon. However, in City Folk, the two fish are separated, with the Salmon in the left tank, and the King Salmon in the right.
Donating to the Museum
In Animal Crossing
"Even run-of-the-mill common fish are valuable resources, you see. All part of the grand tapestry, wot! We will take excellent care of this fellow, oh yes. You can rest assured. Excellent care, indeed."
The Salmon can be found in the back with the ocean fish of the fish exhibit.
In Wild World
"Salmon grilled over hot coals... Delicious. I say, grilling and eating outdoors simply cannot be beat, eh wot?"
The Salmon can be found in the first room of the fish exhibit in the second tank.
In City Folk
"Ahh... A salmon? Yes, quite! Everyone knows that salmon swim upstream to spawn... But did you know they don't eat for the entirety of their journey? Indeed! Salmon must enjoy pushing themselves to the limit... I hereby dub them the pro athletes of the river!"
The Salmon can be found in the large left tank on the first level of the fish exhibit.
In New Leaf
"Salmon are popular migratory fish that are born in rivers, migrate to oceans, and return in four years. In Japan, flavored salmon eggs are a very popular dish called ikura, but it hasn't caught on in the US. In rare cases, salmon will only stay in the ocean for one or two years before coming back. These fish are known as 'phantom fish' since they're so hard to find in the wild. Their characteristic pink color comes from the shrimp and krill they get in their diet. If they are differently, their flesh would be white."
Once the player has caught a particular fish or bug, they can find information about it in the bug or fish menus. Once the Salmon has been caught, the following information can be found in the fish menu:
The Coho salmon can be found in the Pacific Ocean and rivers that run into the north pacific, including California and Russia. Most of the fish (other than the ones stocked into the great lakes) live in the ocean, except when it is time to come to freshwater, where there are less predators, so the fish can reproduce safe in the knowledge that there is less chance of the offspring being eaten. Unfortunately, they always die after the first spawn. The fish does not require salt water to live, however. Nearly 40% of young salmon are eaten by trout, such as the rainbow trout.